1: Summary Math disability affects about 6% of the childhood population and continues into adolescence, and therefore, represents a major biomedical issue. Neuropsychological and neuroimaging studies in adults have implicated the intra-parietal sulcus and middle frontal gyrus in quantity comparison (e.g. subtraction), but have implicated temporo- parietal cortex and inferior frontal gyrus in verbal retrieval of math facts (e.g. multiplication). Neuroimaging research in adults has also suggested that reliance on the verbal retrieval system increases with practice and it is utilized more for easier problems. Behavioral research suggests there are developmental increases in the reliance on the verbal retrieval system and that some children with math disability show pronounced deficits in this system. Behavioral research has also shown that some children with math disability have a reading disability, and some have suggested that children with a math and reading disability (MD+RD) should have a deficit in the verbal retrieval system, whereas children with a math disability only (MD) should have a deficit in the quantity comparison system. Although there has been a substantial amount of neuroimaging research in adults and behavioral research in children, very little is known about the neural basis of math development or of math disability in children. This project will use functional magnetic resonance imaging (fMRI) to examine the neural development of quantity comparison (i.e. subtraction) and verbal retrieval (i.e. multiplication) in typically developing children (10- to 14-year-olds) and to examine the neural bases of math disability in children with and without co-morbid reading disability. We will examine differences in signal intensity using conventional fMRI analyses, but we will also examine differences in effective connectivity using dynamic causal modeling (DCM). This project will employ easier versus harder tasks to more effectively delineate brain regions involved in different forms of math processing, a longitudinal design to examine whether early functioning predicts later development, and reading tasks to determine their inter-relations with math processing. Relevance. This project proposes to examine the neural basis of mathematical development in typically developing children and in children math disability with and without co-morbid reading disability. The results of this project will have implications for the diagnosis and remediation of math disability.